Synchronizing the transmitting and receiving hardware is a necessary step in achieving reliable, quality communications in wireless systems. The synchronization (sync) process includes frequency synchronization and timing synchronization. Frequency synchronization involves measuring and compensating for the difference in frequency between the transmitting hardware's oscillator and the receiving hardware's oscillator. Timing synchronization involves adjusting the receiver's decimation phase such that the ensuing demodulation process occurs at prespecified baud boundaries. Improper frequency synchronization results in a frequency offset in the received signal, while improper timing synchronization may result in intersymbol interference (ISI). In either case, large errors in synchronization may lead to unreliable and poor quality communications.
In single carrier digital communication systems, achieving proper synchronization is fairly straightforward and many solutions exist. In multicarrier, or orthogonal frequency division multiplexed (OFDM), systems, achieving accurate synchronization is more critical because synchronization errors may lead to not only ISI, but also inter-carrier interference (ICI). Moreover, while many OFDM systems utilize a guard interval in order to combat ISI due to channel multipath distortion, the guard interval may lead to ambiguity in the timing synchronization process.
A guard interval consists of a cyclic extension of an OFDM baud and is intended to absorb the multipath distortion in the channel and provide for one or more ISI-free sampling points. The receiver may adjust its decimation phase, allowing any samples in the original baud corrupted by ISI to be “replaced” by samples in the guard interval during demodulation. Baud boundary ambiguity arises because of the possible presence of more than one ISI-free sampling point. Adjusting the decimation phase to include samples from the guard interval may lead to phase rotation between successive OFDM subcarriers after demodulation, i.e., a subcarrier rotation offset. If ignored, this sampling phase-induced subcarrier rotation may cause channel estimation problems.
Accordingly, there is a need for a method of achieving synchronization in OFDM systems that is spectrally efficient and corrects undesirable subcarrier rotation.